1. Field of the Invention
The present invention relates to a sensor control apparatus for controlling a gas sensor element which outputs a detection signal that changes in accordance with air-fuel ratio while making use of exhaust gas of an internal combustion engine, and to an air-fuel ratio detection apparatus using the sensor control apparatus.
2. Description of the Related Art
In recent years, air-fuel ratio control of an internal combustion engine such as a gasoline engine has been performed by use of an air-fuel ratio control apparatus, which includes a gas sensor element capable of detecting, in a wide range, air-fuel ratio of a gas mixture taken into the engine, and a sensor control apparatus for controlling the element, in order to meet demand for enhancing control accuracy and demand for lean burn operation. Meanwhile, in order to cope with strengthened emission control of internal combustion engines and demand for further improved fuel consumption, there arises a strong demand for improving controllability in stoichiometric burn control in which air-fuel ratio is feedback-controlled in the vicinity of the stoichiometric air-fuel ratio, lean burn control in which air-fuel ratio is feedback-controlled in a predetermined lean region, or a like control.
Under such circumstances, Patent Documents 1 to 3 propose air-fuel-ratio control apparatuses each including a gas sensor element formed of a solid electrolyte body and outputting a detection signal which changes in accordance with air-fuel ratio.
Among these documents, Patent Document 1 discloses an air-fuel ratio detection apparatus which uses two differential amplifiers of different amplification factors; i.e., a differential amplifier whose amplification factor is 1 fold and a differential amplifier whose amplification factor is 5 fold, as amplification means for amplifying an output voltage corresponding to pump current flowing through an oxygen pump element, and which is configured such that when the air-fuel ratio is close to the stoichiometric air-fuel ratio (hereinafter may be simply referred to as “stoichiometric ratio”), an output from the differential amplifier of higher amplification factor is used, and when the air-fuel ratio is away from the stoichiometric ratio, an output from the differential amplifier of lower amplification factor is used. Thus, when the air-fuel ratio is close to the stoichiometric ratio, the air-fuel ratio can be detected more accurately.
Patent Document 2 discloses a gas concentration detection apparatus which similarly uses two amplifiers of different amplification factors; i.e., an amplifier whose amplification factor is 5 fold and an amplifier whose amplification factor is 15 fold, as amplification means for amplifying an output voltage of a current detection resistor connected to a sensor element, and which is configured such that when the air-fuel ratio is close to the stoichiometric ratio, an output from the amplifier of higher amplification factor is used, and when the air-fuel ratio is away from the stoichiometric ratio, an output from the amplifier of lower amplification factor is used. In this gas detection apparatus as well, when the air-fuel ratio is close to the stoichiometric ratio, the air-fuel ratio can be detected more accurately.
Patent Document 3 discloses a gas concentration detection apparatus which includes a current detection resistor for detecting the value of current flowing through a gas concentration sensor, and voltage signal output means for outputting this current value as a voltage signal, and which also includes a switch circuit for switching the resistance of the current detection resistor in accordance with the current value.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H1-152356
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2004-205488
[Patent Document 3] Japanese Patent No. 3487159
3. Problems to be Solved by the Invention
However, according to the techniques disclosed in Patent Documents 1 and 2, there must be prepared amplifiers which are equal in number to desired amplification factors, and therefore, the sensor control apparatus becomes expensive.
Meanwhile, according to the technique disclosed in Patent Document 3, since a voltage signal is obtained while the resistance of the current detection resistor is switched, a signal of an appropriate magnitude is difficult to obtain.
Further, there is a demand for a control apparatus for a gas sensor element which can output three types of signals; i.e., a signal suitable for performing lean burn control, a signal suitable for performing stoichiometric burn control, and a signal which can be detected as an air-fuel ratio in a wide range from a rich region to a lean region.